I've wondered about the same. I think R3 should look great on the 2nd gen but they look almost (if not identical design) as the 2nd gen MS3 rims. IIRC, the three major differences are that the R3 rims are 19”, are forged (!!!), and cost out of this world. If you come across an authentic set (there are known knock-offs) for cheap or the seller is not aware, BUY THEM! RX8 rims across all gens are lighter than MS3 rims across all gens and the better choice.
Agree with everything above. Personally I’d go with the neutered silver look on a red (also a red owner). IMO, dark colors is a bit too racy/ricy on the Mz5 and I think it goes better on a car with hard lines, not banged up curves like the 2nd gen. I will leave it to the design of the rim to make it stand out, not the color. I have a set of silver BBS RK1s and didn’t like them test fitted on my previous silver Mz5; they blended in way too much. Look forward to fixing them for the cooper red.
Funny you mention (neon) green! Looking at the color wheel, green (granted not neon) is the complementary color to red. I think you can rock hunter green (j/k). Cooper or bronze would look nice on red, I think.